In a Digital Television (DTV) system, the signal at the receiver often includes distortions introduced by the transmitter, the transmission channel and/or the receiver itself. Consequently, DTV receivers normally include an equalizer to remove linear distortions. The equalizer may be an adaptive equalizer, i.e., one which employs an equalizer adaptation method that is responsive to the differences (“error information”) between the equalizer's output and the transmitted DTV signal. The error information is calculated by subtracting the equalizer output from the received signal. An adaptive equalizer typically has taps with tap weights (coefficients). Adapting the equalizer involves calculating and repeatedly recalculating the tap coefficients.
The DTV signal reception process can be divided into two phases: signal acquisition and signal tracking. During the tracking phase, which is the phase after the system has solidly acquired the DTV signal, equalizer adaptation is “blindly” maintained by the use of Viterbi decoder “soft decisions”. Soft-decision Viterbi decoders maintain a history of many possible transmitted sequences, building up a view of their relative likelihoods and finally selecting the value of logic 0 or 1 for each bit according to which has the maximum likelihood. Viterbi soft decisions are 8-VSB constellations which are mapped from the corresponding Viterbi decoded bits.
During the acquisition phase, which is the period of time when Viterbi decoder decisions are not yet reliable, a training sequence is often used to initiate the adaptive equalizer. For example, the 8-VSB Advanced Television Systems Committee (ATSC) signal employed by the United States' ATSC digital television system includes a Data Field Sync (DFS) training signal, whose length is 820 symbols. This DFS training signal is repetitively transmitted every 313 DTV segments. Prior art solutions employ the DFS training signal to initiate the adaptive equalizer during the training signal period. However, in the presence of severe multi-path conditions, the training signal period is often too short for the equalizer to converge to a correct solution. This results in an unsuccessful transition between the acquisition phase and the tracking phase using Viterbi decoder soft decisions to drive equalizer adaptation.
There are two major approaches to carrier/phase synchronization: According to the first approach, carrier synchronization is done before equalization. In prior art solutions, the phase error upon which the carrier is tracked includes terms relating to all signal paths, i.e., the main path and all ghost paths. The main path is the specific signal path to which the equalizer aims to synchronize. A ghost path is any signal path other than the main path. As a result, in strong multi-path conditions, the channel impulse response (CIR) portrayed to the equalizer may be very hard to cancel.
According to the second approach, the phase tracking error is based upon the equalizer's output, which means that its integrity depends on the equalizer's convergence, and so in strong multi-path conditions, this approach can be used for tracking only.